Seasonal and tidal variability of the hydrology and suspended particulate matter in the Van Uc estuary, Red River, Vietnam

This study explores the seasonal and tidal changes in flow and suspended matter dynamics in the tropical and macrotidal Van Uc River estuary (North Vietnam) and aims at understanding, among others, the sediment delivery to the ocean and the processes behind the estuarine siltation. Four campaigns took place during the contrasting high flow and low flow seasons, for each during neap and spring tides. Water and suspended matter fluxes, salinity, turbidity and particle size distribution (PSD) were measured for 24 h at three cross sections along the estuary. During the high-flow season, the estuary was mixed, with seaward sediment flux during neap tide and ephemeral up-estuary sediment flux at spring tides due to tidal distortion. During the low-flow season, the system transitioned from partially mixed to highly stratified with salty waters intrusions, salt wedge presence and up-estuary sediment flux varying with the tidal regime. The highest up-estuary sediment flow occurred during low-flow at spring tides and represented 2/3 of the outflow. The lower river discharge, the higher PSD variation. The increasing proportion of microflocs with increasing salinity as compared to primary particles and flocculi was observed at both seasons. Higher tidal variations of PSD at spring tides indicate the strong effect of turbulent shear. These findings also provide a new benchmark for hydro-sedimentary model calibration/validation.